1. Field of the Invention
The present invention relates to an anti-reflective material having particularly excellent display stainproofing properties, durability anci anti-reflective properties, which is suited for use in displays such as CRTS, LCDs, and PDPs.
2. Description of Related Art
With the recent development of displays such as CRTs, LCDs, and PDPs, various types of equipment incorporating these displays are used in various fields. When using such equipment in places exposed to comparatively bright light such as outdoors, problems such as reflections from external light such as sunlight and light from fluorescent lamps in the display have increased. Therefore, it is strongly required to prevent the reflection of external light on display surfaces, that is, to prevent reflection.
Therefore, display surface are frequently provided with anti-reflective films to improve their visibility. Such anti-reflective films are generally formed by providing the surface with a material having a refractive index lower than that of a lower layer in an optical film thickness which is n/4 times (wherein n is an odd number) as large as the wavelength of the light of interest. As the method of forming this outermost layer, for example, there can be used a method of forming the layer by deposition or sputtering of materials such as magnesium fluoride and silica, a method of coating a coating solution prepared by dispersing these materials, and a method of forming a fluorine-based polymer film by deposition and coating.
However, the anti-reflective film according to the above prior art has the problems that adhesion of dust or oily substances in the atmosphere to the surface of the substrate or adhesion of fingerprints is liable to quickly show as soiling. For example, stains on the surface of transparent glass or plastic impairs the appearance of display window glass, impairs the field of view in case of eyeglasses, and prevents safe driving in case of vehicle windows. Furthermore, for optical instruments such as cameras, stains impair their performance. Therefore, there has been demand for an anti-reflective film to which stains do not readily adhere, and from which stains can be easily removed once they adhere.
To prevent stains, various methods have hitherto been suggested. For example, Japanese Unexamined Patent Application, First Publication No. Sho 58-142958, Japanese Unexamined Patent Application, First Publication No. Sho 58-147483, and Japanese Unexamined Patent Application, First Publication No. Sho 58-147484 disclose that a water- and oil-repellent agent for glass surfaces, comprising a polyfluoroalkyl group-containing silane compound or a partially hydrolyzed condensate thereof, water-glass or a silane coupling agent and colloidal silica, is effective. Japanese Unexamined Patent Application, First Publication No. Sho 60-49079 discloses a fingerprint adhesion inhibitor comprising a compound having a C4-21 polyfluoroalkyl group or a polyfluoroether group. Japanese Unexamined Patent Application, First Publication No. Hei 4-89877 discloses a protective water-repellent composition for coating of vehicles, comprising an organic solvent-soluble organopolysiloxane resin partially having a perfluoroalkyl group and a lubricant oil which is insoluble in water. Japanese Unexamined Patent Application, First Publication No. Hei 5-125083 discloses a method of preparing a surface treating agent by reacting a tetraalkoxide of Ti or Zr with a fluorine-containing alkyl silane compound. Furthermore, Japanese Unexamined Patent Application, First Publication No. Hei 9-157582 discloses a stainproofing base material comprising a base material, and a layer of a silicon-containing organic fluoropolymer with a specific structure having a perfluoropolyether group which is formed on the surface of the base material.
In Japanese Unexamined Patent Application, First Publication No. Hei 9-127307, an anti-reflective filter coated with perfluoroether having a polar group at a terminal and a display device are suggested for stainproofing properties of a single-layer or multi-layer anti-reflective film comprising as a principal component silicone dioxide formed on a transparent substrate by a vacuum deposition or sputtering method, as an example wherein special attention is paid to use for displays. Furthermore, a stainproofing film containing alkoxysilane compound with a specific structure having a perfluoropolyether group is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-255919 and a surface modified film and a filter for display device, which are obtained by using the alkoxysilane compound having a perfluoropolyether group in combination with various materials are disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-326240, Japanese Unexamined Patent Application, First Publication No. Hei 10-26701, Japanese Unexamined Patent Application, First Publication No. Hei 10-120442, and Japanese Unexamined Patent Application, First Publication No. Hei 10-148701.
However, the above-mentioned prior arts with respect to the stainproofing properties have the following problems. That is, when various fluorine-containing compounds having stainproofing properties for an outermost layer of the anti-reflective material are applied to a low-refraction index layer made of silicon dioxide formed by a deposition or sputtering method, the anti-reflective properties are impaired and the stainproofing properties, particularly the durability in stainproofing properties, are poor. The reason is that the stainproofing effect is reduced over time, though excellent stainproofing properties are retained shortly after coating of the fluorine compound. It has also been found that conventional stainproofing measure have the problem that, when stains adhering to a base material coated with a stainproofing component are wiped off repeatedly, the stainproofing component is also wiped off, and the stainproofing properties are reduced, namely, contact adhesion between the low-refractive index layer and the stainproofing coated film is poor.
Thus, an object of the present invention is to provide an anti-reflective material which is provided with a stainproofing layer having excellent durability of the stainproofing effect without exerting an adverse effect on the stainproofing capability.
The present inventors have carried out intensive studies to improve the stainproofing properties of the display surface, thus attaining the above object by the following method. That is, the present invention provides an anti-reflective material comprising a transparent substrate and at least a low-refractive index layer and a stainproofing layer laminated in order on said transparent substrate, wherein said low-refractive index layer is a silica film formed by the sol-gel method, and said stainproofing layer contains at least a perfluoroalkyl ether compound. The present invention also provides an anti-reflective material film using, as the perfluoroalkyl ether compound, a compound having one or more functional groups having high affinity with silica and/or chemically bonding functional groups at the molecular terminal.
The anti-reflective material of the present invention can easily exhibit excellent stainproofing properties, durability, and anti-reflective properties when used as a display.
The stainproofing layer in the present invention contains at least a perfluoroalkyl ether compound, and the compound serves as a stainproofing component which substantially exhibits stainproofing properties and a compound having a perfluoroalkyl ether group is appropriately used. Among these compounds, a compound having one or more functional groups having high affinity with a silica film described hereinafter and/or chemically bonding functional groups at the molecular terminal is preferable. Examples of the perfluoroalkyl compounds are compounds represented by the following formulae 1 and 2.
(In the formulae, Rf represents a perfluroroalkyl ether group, R1 and R2 represent a divalent organic group, X1 and X2 represent a monovalent group having a functional group having high affinity with silica and/or a chemically bonding functional group, and R1 and R2, X1 and X2 may be the same or different).
R1 and R2in the above formulae are not particularly limited. However, examples thereof include xe2x80x94R3xe2x80x94, xe2x80x94R3xe2x80x94COOxe2x80x94R4xe2x80x94, xe2x80x94R3xe2x80x94CONHxe2x80x94R4xe2x80x94, and xe2x80x94R3xe2x80x94Oxe2x80x94R4xe2x80x94 (wherein R3 and R4 represent a direct bond, a C1 to C6 alkylene group, a phenylene group, a naphthalene group, or a combination of these, and R3 and R4 may be the same or different). In addition, the above alkylene, phenylene, and naphthalene groups may have a substituent group.
The molecular structure of the perfluoroalkyl ether group is not specifically limited, but a monovalent or divalent perfluoroalkyl ether group containing a C1-C3 perfluoroalkyloxy group as a repeating unit is preferable, and specific examples thereof include the following. In the following chemical structure, n and m represent an integer of 1 or more. In addition, in the formula having both n and m, either n or m may be 0. 
It is said that functional groups such as hydroxyl group and silanol group are exposed on the surface of the silica film constituting the low-refractive index layer. Accordingly, it is effective that the stainproofing component having a perfluoroalkyl ether group have one or more functional groups having high affinity with silica and/or chemically bonding functional groups at the molecular terminal so as to have a high cohesive force with the silica film. Functional groups which have a high affinity are those groups capable of cohering by an ionic bond or a van der Waals force, and specific examples thereof include carboxyl groups, carboxylate groups, phosphoric groups, phosphate groups, sulfonic groups, sulfonate groups, hydroxyl groups, amino groups, imino groups, ammonium salt groups, amide groups, and thiol groups. The chemically bonding functional groups include, for example, isocyanate groups capable of bonding with the hydroxyl group of the silica film, chlorosilyl groups capable of bonding with a silanol group, and alkoxysilyl groups. To the silica film constituting the anti-reflective material of the present invention, carboxyl groups, isocyanate groups, and alkoxysilyl groups are preferable, and alkoxysilyl groups are particularly preferable. The perfluoroalkyl ether compound preferably has one or more functional groups in the molecule, and can have same or two different functional groups in the case of a divalent perfluoroalkyl ether group.
The perfluoroalkyl ether group and functional group having high affinity with silica and/or chemically bonding functional group are bonded directly or bonded through a proper divalent atom or group. However, these molecular structures are not specifically limited and can be selected according to the ease of synthesis. The bonding group includes, for example, atoms or atomic groups such as O, NH, and S., and alkylene groups, phenylene groups, alkyl ether groups, carbonyl groups, ester groups, and amide groups. These bonding groups can be used alone or in combination.
Examples of the perfluoroalkyl ether compound which can be used in the present invention are listed as follows. 
The perfluoroalkyl ether compound used in the present invention is not limited thereto, and two or more kinds can be used in combination. Furthermore, the molecular weight of these compounds is preferably within the range of 500-10000, and more preferably within the range of 500-4000. When the molecular weight is 500 or less, sufficient durability of stainproofing properties is not exhibited. On the other hand, when the molecular weight is 10000 or more, the solubility in a solvent is lowered, thereby making it difficult to form a uniform stainproofing layer.
The above compound itself shows a low refractive index because it contains many fluorine atoms in the molecule. Accordingly, a reduction in reflectance can also be expected without inhibiting the anti-reflective performance attained by the presence of the low-refractive index layer as the lower layer.
The stainproofing layer in the present invention is formed by coating and drying a solution prepared by dissolving at least the compound described above in the solvent. The coating weight of the compound is preferably within a range of 0.1-50 nm, and more preferably within a range of 0.5-20 nm, in terms of the thickness of the stainproofing layer. When the thickness is 0.1 nm or less, the function of the stainproofing properties is not exhibited. On the other hand, when the thickness exceeds 50 nm, sufficient stainproofing properties are obtained, but the coated surface is slightly sticky, and it is not economical. To prepare a coating composition for forming the stainproofing layer in the present invention, the concentration of the solution of a stainproofing active component is first decided considering the composition, coating method, and coated film thickness of a silica film, and then, based on the concentration, a fluorine-based solvent is selected, and an composition ratio is determined.
It is necessary that the solvent used in the coating composition for forming the stainproofing layer have sufficient solubility to form a uniform stainproofing layer and a boiling point suitable for coating. Solvents capable of dissolving the perfluoroalkyl ether compound as the stainproofing component include halogen-based solvents and fluorine-based solvents, but are preferably selected fully considering safety to the human body, lifetime in the atmosphere, ozone depletion potential, global warming potential, and possibility of being a cause of photochemical smog. From such a point of view, hydrofluorocarbons (HFC) and hydrofluoro ethers (HFE) are preferably used as the fluorine-based solvent in the present invention. HFC is basically composed of hydrogen, fluorine, and carbon atoms, while HFE further contains oxygen atoms in ether groups in the molecule. These solvents include, but are not limited to, xe2x80x9cVERTREL XFxe2x80x9d (CF3CHFCHFCF2CF3: bp. 55xc2x0 C.) which is a trade name of Du Pont Co.; xe2x80x9cZEORORA Hxe2x80x9d (heptafluorocyclopentane: bp. 82.5xc2x0 C.) which is a trade name of Nippon Zeon Co., Ltd.; and xe2x80x9cHFE-7100xe2x80x9d (C4F9OCH3: bp. 60xc2x0 C.) and xe2x80x9cHFE-7200xe2x80x9d (C4F9OC2H5: bp. 78xc2x0 C.) which are trade names of 3M Co. These solvents are excellent solvents, which show high safety because they are free from halogen atoms other than fluorine and show zero ozone depletion potential as compared with other fluorine-based solvents as shown in the following table, and which show global warming potential smaller than that of a conventional fluorine-based solvent by one order of magnitude and cause no photochemical smog.
In the present invention, one or more solvents selected from these solvents can be used alone or in combination. Since these fluorine-based solvents have a cost higher than that of a conventional solvent by one or more orders of magnitude, the cost can also be reduced by diluting with an inexpensive non-fluorine-based solvent on use.
The non-fluorine-based solvents used in the present invention may be those which are compatible with the above fluorine-based solvent among conventional solvents used in normal coatings and which serve as so-called diluents for reducing the amount of an expensive fluorine-based solvent. These solvents themselves hardly dissolve the perfluoroalkyl ether compound, but the solubility can be improved by mixing with the fluorine-based solvent without impairing the excellent solubility of the fluorine-based solvent. Specifically, there can be used alcohol-based solvents such as methanol, ethanol, isopropanol (IPA), and sec-butyl alcohol; ketone-based solvents such as acetone, methyl ethyl ketone (MEK), and methyl isobutyl ketone (MIBK); ester-based solvents such as ethyl acetate, isopropyl acetate, and butyl acetate; ether-based solvents such as diisopropyl ether, dibutyl ether, tetrahydrofuran (THF), methyl cellosolve, and ethyl cellosolve; aliphatic hydrocarbon-based solvents such as hexane, cyclohexane, methylcyclohexane, heptane, and octane; and aromatic hydrocabon-based solvents such as benzene, toluene, and xylene. These non-fluorine-based solvents can also be used alone or in combination.
As the fluorine-based and non-fluorine-based solvents in the present invention, those having a boiling point of 50 to 150xc2x0 C. are preferable. When the boiling point is 50xc2x0 C. or lower, unevenness of coating is liable to occur because of excessive volatility of the solvent, and the operation environment becomes inferior. On the other hand, it is economically disadvantageous to use a solvent having a boiling point of 150xc2x0 C. or higher because drying it requires much energy and time, and the solvent can sometimes not be applied to a plastic film having poor heat resistance. Furthermore, a difference in boiling point between the fluorine-based solvent and non-fluorine-based solvent is preferably 50xc2x0 C. or less. The reason is as follows. When the difference in boiling point between both solvents is larger than 50xc2x0 C. or more, the evaporation behavior of the solvent on coating and drying differs so that defects such as unevenness of the coating occur.
When using a mixed solvent in the present invention, the amount of the fluorine-based solvent contained in the mixed solvent is preferably 1% by weight or more. When the amount of the fluorine-based solvent is 1% by weight or less, it becomes impossible to dissolve the stainproofing agent in a sufficiently stable manner.
The optimum amount of the perfluoroalkyl ether compound contained in the coating composition for the stainproofing layer in the present invention cannot be unconditionally decided because it varies depending on the low-refractive index layer and coating method described hereinafter, but is preferably within a range of 0.01-10% by weight, and more preferably within a range of 0.03-1% by weight. When the amount is 0.01% by weight or less, sufficient stainproofing properties cannot be imparted to the stainproofing layer even if when coated with the resulting coating composition. On the other hand, when the amount exceeds 10% by weight, stickiness occurs, due to an excess of the stainproofing component, and it is economically disadvantageous.
In the present invention, various materials can be added to the stainproofing layer, in addition to the above materials, for the purpose of further enhancing the stainproofing properties, lowering the frictional force, enhancing the abrasion resistance and improving the wettability with the low-refractive index layer. Specific examples thereof include various silane coupling agents, titanate coupling agents, silicone oils, silicone resins, and surfactants.
Usable silane coupling agents include, for example, vinyltrichlorosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(xcex2-methoxyethoxy)silane, vinyltriacetoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldiethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, xcex3-methacryloxypropylmethyldimethoxysilane, N-(xcex2-aminoethyl)-xcex3-aminopropyltrimethoxysilane, N-(xcex2-aminoethyl)-xcex3-aminopropylmethyldimethoxysilane, xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltriethoxysilane, N-phenyl-xcex3-aminopropyltrimethoxysilane, xcex3-mercaptopropyltrimethoxysilane, xcex3-chloropropyltrimethoxysilane, xcex3-chloropropylmethyldimethoxysilane, and xcex3-chloropropylmethyldichlorosilane. A silane coupling agent having a perfluoroalkyl group can also be used.
Titanate coupling agents include, for example, isopropylisostearyl titanate, isopropyltris(dioctyl pyrophosphate)titanate, tetraoctylbis(ditridecyl phosphate)titanate, tetra(2,2-dialkyloxymethyl-1-butyl)bis(ditridecyl)phosphate titanate, bis(dioctyl pyrophosphate) oxyacetate titanate, and isopropyltri (N-aminoethyl-aminoethyl) titanate.
Silicone oils and silicone resins can be selected from general lubricating and releasing silicone oils and resins. Considering the cohesive force with the low-refractive index material, those having functional groups such as epoxy groups, alkyl groups, amino groups, carboxyl groups, and alcohol groups at the molecular side chain or terminal are preferable. Furthermore, fluorine-modified products can also be used. To improve the wettability with the low-refractive index layer and to prevent the unevenness of coating, various surfactants can be used, and anionic and nonionic surfactants are preferable.
Surfactants having a perfluoroalkyl group are preferable because they enhance the solubility of the perfluoroalkyl ether compound as the stainproofing component of the present invention.
The amount of these additives is preferably 30% by weight or less, and more preferably 10% by weight or less, based on the perfluoroalkyl ether compound. When the amount is 30% by weight or more, the stainproofing properties are lowered, and it becomes difficult to coat the coating composition uniformly, thereby impairing excellent stainproofing properties of the present invention.
The low-refractive index layer in the present invention is a silica film formed by the sol-gel method in view of improvement in contact adhesion with the stainproofing layer. The sol-gel method refers to a method of hydrolyzing and polycondensing an alkoxysilane such as tetraethoxysilane (TEOS), tetramethoxysilane, and terabutoxysilane using an acid catalyst to prepare a silica sol, coating with the silica sol, followed by heating and further drying to form a silica film. As the acid catalyst, for example, there can be used hydrochloric acid, sulfuric acid, nitric acid, acetic acid, hydrofluoric acid, phosphoric acid, oxalic acid, phthalic acid, and maleic acid. In the above film forming method, alcohols such as methanol, ethanol, propanol, and butanol; ethylene glycol, hexyl glycol, triethanolamine, and xylene can be used as the solvent. Excellent durability of the stainproofing layer can be attained only by forming such a low-refractive index layer and forming the above stainproofing layer thereon, and sufficient durability of the stainproofing properties cannot be obtained when the silica layer is a silica film formed by the deposition method or sputtering method. The reason is as follows. The number of chemical bond sites with the perfluoroalkyl ether compound of the silica film obtained by the sol-gel method is larger than that of the silica film obtained by the deposition or sputtering method so that the contact adhesion with the stainproofing layer is improved. It is considered that there is also an effect exerted by the difference in surface shape between the silica film obtained by the sol-gel method and the silica film obtained by the deposition or sputtering method, but the details are unclear. To improve the contact adhesion and to lower the refractive index, various silane coupling agents, colloidal silica, magnesium fluoride sol, and alkylalkoxysilane fluoride are preferably added to the sol-gel coating composition, in addition to the alkoxysilane such as TEOS.
The anti-reflective material of the present invention is formed by laminating at least a low-refractive index layer and a stainproofing layer in order on a transparent substrate. As the transparent substrate, for example, there can be used plates or films made of glass or plastic which have widely been used in homes, offices, automobiles, and displays. As the plastic film, for example, PET (polyethylene terephthalate) and TAC (triacetylcellulose) are preferable in view of the mechanical strength and optical transparency. It is said that the glass has silanol groups on its surface. It is considered that PET and TAC have functional groups such as hydroxyl groups, carboxyl groups, ester groups, and aromatic rings on its surface.
In the present invention, a hard coat layer can be optionally formed between the transparent substrate and low-refractive index layer. The hard coat layer is cured by radiation and/or heat. Using general hard coat materials such as acrylic-, silicone-, and acrylic-silicone-based materials, the hard coat layer can be formed on the transparent substrate. Specific examples thereof include, but are not limited to, xe2x80x9cARONIX UV-3700xe2x80x9d manufactured by Toagosei Co., Ltd., and xe2x80x9cUVHC8553xe2x80x9d manufactured by Toshiba Silicone Co., Ltd. It is preferable that these materials have hard coating properties with a pencil hardness of 3H or higher (JIS K5400), and the coating thickness is within a range of 0.5-10 xcexcm. When the thickness is 0.5 xcexcm or less, sufficient hard coating properties are not exhibited. On the other hand, a thickness of 10 xcexcm or more is not preferred because cracks occur. For the purpose of lowering the reflectance and improving the contact adhesion with the transparent substrate, various materials may be added as far as the hard coating properties of the resin are not lowered.
In the anti-reflective material of the present invention, the transparent substrate can be coated with a coating composition for each layer using various methods used in a conventional coating operation. Usable coating methods include, but are not limited to, the spin coating method, dipping method, flood coating, gravure method, flexo method, Mayer bar method, microgravure method, spray method, kiss-roll method, air knife method, blade method, and three-roll method, and the optimum method is preferably selected considering the kind, shape, and size of the coating materials, the coating compositions, and the physical properties. The drying conditions must be optimized considering the vapor pressures and boiling points of the solvents to be used. Since the mixed solvent of the coating composition for the stainproofing layer used in the present invention has a boiling point within a range of 50 to 150xc2x0 C., and the coating weight of the stainproofing layer required to obtain sufficient stainproofing performance is considerably small, the solvent can be dried comparatively easily. To firmly bond the stainproofing layer to the low-refractive index layer, the curing conditions must be optimized according to the type of functional group of the perfluoroalkyl ether compound. In case where the perfluoroalkyl ether compound has an alkoxysilane at the terminal, and the stainproofing layer is chemically bonded with the low-refractive index layer through a silanol bond, the material is preferably heated at 100-200xc2x0 C. for several hours or allowed to stand in a high-humidity atmosphere for several days to several weeks. When using a stainproofing component having an isocyanate group at the terminal, curing is preferably conducted at 30-70xc2x0 C. for several days to several weeks. To improve the contact adhesion between the respective layers and to improve the wettability of the coating composition, the substrate surface or surface to be coated can be subjected to a plasma or corona treatment.